Cellphone footage showed security officers on April 9 dragging Dr. David Dao by his arms and legs down the aisle and off the Louisville, Kentucky-bound flight before it took off at Chicago O'Hare International Airport.

Dao was left bloodied and bruised in the incident that created a firestorm for the airline.

Three Chicago Department of Aviation security officers and a sergeant "mishandled a nonthreatening situation that resulted in aphysically violent forcible removalof a passenger" aboard the flight, a City of Chicago Office of Inspector General investigation found, officials said Tuesday.

Employees made "misleading statements and deliberately removed material facts from their reports," the investigation found.

The aviation department, acting on the inspector general's findings and recommendations, fired the officer "who improperly escalated the incident," the inspector general's office said. The sergeant "involved in the deliberate removal of facts from an employee report" also was terminated, officials said.

Two officers were suspended, officials said. None of the four was named.

"The use of excessive force caused the passenger to hit his face on an armrest, resulting in a concussion, a broken nose and the loss of two teeth," officials said.

Dao's attorney: 'Lesson to be learned' for officers

In a statement, Dao's attorney, Thomas Demetrio, said: "It is unfortunate the conduct of these two city aviation employees has resulted in their losing their jobs."

"There is a lesson to be learned here for police officers at all levels. Do not state something that is clearly contrary to video viewed by the world," Demetrio said. "But for the video, the filed report stating that only 'minimal' force was used would have been unnoticed."

United Airlines settled lawsuit by passenger

The incident began when gate agents asked volunteers to give up their seats for a United crew that needed to meet another flight.

Dao and his wife initially agreed to take a later flight, a fellow passenger told CNN, but the couple changed their minds when they learned the next flight wouldn't leave until the following day.

United had offered compensation to anyone willing to give up their seats; Dao apparently was chosen at random when not enough people volunteered.

In a video shot by two passengers sitting behind Dao, he repeatedly refused to get off the flight, telling officers that he is a physician and must work in the morning.

Joya and Forest Cummings told CNN that Dao was not belligerent and got only mildly upset when a second officer arrived and demanded he leave the plane, they said. Dao never raised his voice, the couple said.

An officer was placed on leave after the incident.

Incident reports from the officers said Dao was swinging his arms and had his fists balled as they tried to pull him from his seat.

CEO Oscar Munoz later apologized, calling the encounter "truly horrific." Munoz pledged a "thorough review." Several weeks after the incident, United reached a settlement with Dao.

The dustup had repercussions across the industry.

Southwest announced it would no longer overbook flights, in an attempt to ensure ticket-holding customers will never be booted off flights. Delta said it would offer volunteers up to $10,000 to give up their seats on overbooked flights.